Sonepur is one of the 243 state assembly constituencies in Bihar. It is located in the Saran district of the state. The current MLA from this constituency is Dr. Ramanuj Prasad of the Rashtriya Janata Dal (RJD), who secured 73,247 votes in the 2020 elections, beating Vinay Kumar Singh of the Bharatiya Janata Party (BJP)  by a margin of 6,686 votes.

In the last legislative assembly elections in 2020, the Election Commission documented the total number of registered electors at 288,143, of which 152,564 electors were male, and 135,578 were female and  and one elector was of the third gender. Voter turnout in this election cycle was 59.01% last time.
2024 Election Commission data shows the total number of registered electors in Sonepur constituency at 298,216 electors. Male electors in this constituency amount to 156,758, and female electors are enumerated at 141,456 and 2 electors of the third gender. Polling for this constituency took place in the first phase of elections on November 6, 2025.

Sonepur is famous for the largest cattle fair in Asia which has its origin in ancient times as it is known that Chandragupta Maurya used to buy horses and elephants from this region. The Hariharnath Mandir is also in Sonepur which is a famous temple and the site of the Battle of Ghaghra.
This constituency covers an approximate land area of 238.03 square kilometers, Sonepur shares its borders with seven other legislative assembly constituencies of Bihar including Parsa, Hajipur, Garkha, Maner, Danapur, Digha and Bankipur.